1. Field of the Invention
The present invention relates to a micromirror unit used in optical apparatus for the purposes of changing the direction of light. In particular, it relates to a micromirror unit of the type which is advantageously incorporated in an optical disk apparatus (for writing to or reading data from an optical disk), an optical switching apparatus (for selectively connecting one optical fiber to another to provide a light passage), etc. The present invention also relates to a method of making such a micromirror unit.
2. Description of the Related Art
A micromirror unit is provided with a reflective mirror member which is pivotable for changing the direction of reflected light. A popular technique for actuating the mirror member is to utilize electrostatic force. Micromirror units of this type (referred to as “static driving type” hereinafter) may have several structures. Such micromirror units are generally classified into two groups, depending on fabrication methods. One of the methods employs a “surface micro-machining” technique, whereas the other employs a “bulk micro-machining” technique. In accordance with the surface micro-machining, patterned material layers in lamination may be formed on a base substrate, thereby providing required components such as a support, a mirror member and electrodes. In this layer forming process, a dummy layer, which will be removed later, may also be formed on the substrate. In accordance with the bulk micro-machining, on the other hand, abase substrate itself is subjected to etching, thereby providing required components such as a frame and a mirror forming base. Then, a mirror member and electrodes may be formed on the etched substrate by a thin-film forming technique. A conventional micromirror unit of the static driving type by the surface micro-machining is disclosed in JP-A-7(1995)-287177 for example. Other micromirror units of the static driving type by the bulk micro-machining are disclosed in JP-A-9(1997)-146032, JP-A-9(1997)-146034, JP-A-10(1998)-62709 and JP-A-2000-13443.
One of the technically significant factors desired in a micromirror unit is a high flatness of the reflective mirror member. According to the above-mentioned surface micro-machining technique, however, the thickness of the resulting mirror member is rendered very small, so that the mirror member is liable to warp. To avoid this and ensure a high flatness, the mirror member should be made so small that its respective edges are less than 100 μm in length. In accordance with the bulk micro-machining, on the other hand, a rather thick substrate is processed, thereby providing a sufficiently rigid mirror forming base to support the mirror member. Thus, a relatively large mirror member having a high flatness can be obtained. Due to this advantage, the bulk micro-machining technique is widely used to fabricate a micromirror unit having a large mirror member whose edges are more than 100 μm in length.
FIG. 10 of the accompanying drawings shows an example of conventional micromirror unit fabricated by the bulk micro-machining technique. The illustrated micromirror unit 300 is of the static driving type, and includes a lamination of a mirror substrate 310 and an electrode substrate 320. As shown in FIG. 11, the mirror substrate 310 includes a mirror forming base 311 and a frame 313. The mirror forming base 311 has an obverse surface upon which a mirror member 311a is formed. The mirror forming base 311 is supported by the frame 313 via a pair of torsion bars 312. The mirror forming base 311 has an reverse surface upon which a pair of electrodes 314a and 314b is formed. As shown in FIG. 10, the electrode substrate 320 is provided with a pair of electrodes 321a and 321b which faces the above-mentioned pair of electrodes 314a and 314b of the mirror forming base 311.
With the above arrangement, the electrodes 314a, 314b of the mirror forming base 311 may be positively charged, whereas the electrode 321a of the electrode substrate 320 may be negatively charged. As a result, an electrostatic force is generated between these electrodes, thereby turning the mirror forming base 311 in the N3-direction shown in FIG. 10 as the torsion bars 312 are being twisted. To rotate the mirror forming base 311 in the opposite direction, the other electrode 321b of the substrate 320 may be negatively charged. As readily understood, when the mirror forming base 311 is turned clockwise or counterclockwise, as required, the light reflected on the mirror member 311a is directed in the desired direction.
The conventional mirror substrate 310 is prepared by performing wet etching on a mother substrate (not shown) from one side of the substrate. Accordingly, two identical openings 315 (see FIG. 11) are formed to extend through the thickness of the substrate. Each of the openings 315 has a angular C-like configuration and is arranged in symmetrical facing relation to the other. It should be noted here that the geometry of the mirror substrate 310 is provided only by an etching process.
While the conventional micromirror unit is functional in many respects, it still suffers the following drawback.
When the mirror forming base 311 of the micromirror unit 300 is caused to turn, the rotation angle of the mirror forming base 311 is determined so that the electrostatic force generated between the relevant electrodes balances the restoring force of the twisted torsion bars 312. Therefore, in order to accurately reflect light in a desired direction by the micromirror unit 300, it is necessary to design the respective torsion bars 312 in a manner such that they will exert a prescribed restoring force at a given rotation angle of the mirror forming base 311.
According to the prior art, however, the thickness t1 of each torsion bar 312 is rendered equal to the thickness t2 of the mirror forming base 311. Unfavorably, this design may make it difficult or even impossible to provide each torsion bar 312 with a desired characteristics of torsional resistance against the mirror forming base 311.
According to the prior art, however, the thickness t1 of each torsion bar 312 is rendered equal to the thickness t2 of the mirror forming base 311. Unfavorably, this design may make it difficult or even impossible to provide each torsion bar 312 with a desired characteristics of torsional resistance against the mirror base 311.